This invention relates to capsules and materials for use therein. The capsules of the present invention are especially, but not exclusively, intended for use in electrophoretic displays. This invention also relates to binders for use in electrophoretic displays. This invention also relates to processes for forming electrophoretic media and displays, and to the media and displays so formed.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. (The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element.) Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; and 6,545,291; and U.S. patent applications Publication Nos. 2002/0019081; 2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792; 2002/0154382, 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755; and 2003/0053189; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/20922; WO 00/26761; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; and WO 01/08241.
Known electrophoretic media, both encapsulated and unencapsulated, can be divided into two main types, referred to hereinafter for convenience as “single particle” and “dual particle” respectively. A single particle medium has only a single type of electrophoretic particle suspending in a colored suspending medium, at least one optical characteristic of which differs from that of the particles. (In referring to a single type of particle, we do not imply that all particles of the type are absolutely identical. For example, provided that all particles of the type possess substantially the same optical characteristic and a charge of the same polarity, considerable variation in parameters such as particle size and electrophoretic mobility can be tolerated without affecting the utility of the medium.) The optical characteristic is typically color visible to the human eye, but may, alternatively or in addition, be any one of more of reflectivity, retroreflectivity, luminescence, fluorescence, phosphorescence, or color in the broader sense of meaning a difference in absorption or reflectance at non-visible wavelengths. When such a medium is placed between a pair of electrodes, at least one of which is transparent, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of the particles (when the particles are adjacent the electrode closer to the observer, hereinafter called the “front” electrode) or the optical characteristic of the suspending medium (when the particles are adjacent the electrode remote from the observer, hereinafter called the “rear” electrode, so that the particles are hidden by the colored suspending medium).
A dual particle medium has two different types of particles differing in at least one optical characteristic and a suspending fluid which may be uncolored or colored, but which is typically uncolored. The two types of particles differ in electrophoretic mobility; this difference in mobility may be in polarity (this type may hereinafter be referred to as an “opposite charge dual particle” medium) and/or magnitude. When such a dual particle medium is placed between the aforementioned pair of electrodes, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of either set of particles, although the exact manner in which this is achieved differs depending upon whether the difference in mobility is in polarity or only in magnitude. For ease of illustration, consider an electrophoretic medium in which one type of particles is black and the other type white. If, the two types of particles differ in polarity (if, for example, the black particles are positively charged and the white particles negatively charged), the particles will be attracted to the two different electrodes, so that if, for example, the front electrode is negative relative to the rear electrode, the black particles will be attracted to the front electrode and the white particles to the rear electrode, so that the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, the white particles will be attracted to the front electrode and the black particles to the rear electrode, so that the medium will appear white to the observer.
If the two types of particles have charges of the same polarity, but differ in electrophoretic mobility (this type of medium may hereinafter to referred to as a “same polarity dual particle” medium), both types of particles will be attracted to the same electrode, but one type will reach the electrode before the other, so that the type facing the observer differs depending upon the electrode to which the particles are attracted. For example suppose the previous illustration is modified so that both the black and white particles are positively charged, but the black particles have the higher electrophoretic mobility. If now the front electrode is negative relative to the rear electrode, both the black and white particles will be attracted to the front electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the front electrode and the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, both the black and white particles will be attracted to the rear electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the rear electrode, leaving a layer of white particles remote from the rear electrode and facing the observer, so that the medium will appear white to the observer: note that this type of dual particle medium requires that the suspending fluid to sufficiently transparent to allow the layer of white particles remote from the rear electrode to be readily visible to the observer. Typically, the suspending fluid in a dual particle display is not colored at all, but some color may be incorporated for the purpose of correcting any undesirable tint in the white particles seen therethrough.
Both single and dual particle electrophoretic displays may be capable of intermediate gray states having optical characteristics intermediate the two extreme optical states already described. It is shown in the aforementioned application Ser. No. 10/063,236 that some electrophoretic displays are stable not only in their extreme optical states but also in their intermediate gray states. This type of display is properly called “multi-stable” rather than bistable, but the latter term may be used herein for convenience.
Some of the aforementioned patents and published applications disclose encapsulated electrophoretic media having three or more different types of particles within each capsule. For purposes of the present application, such multi-particle media are regarded as sub-species of dual particle media.
Also, many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned 2002/0131147. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively. Present day electrophoretic displays exhibit paper-like reflective optics, extremely low power consumption due to retained image capability, and mechanical conformability and flexibility.
Although electrophoretic displays are often opaque (since the particles substantially block transmission of visible light through the display) and operate in a reflective mode, electrophoretic displays can be made to operate in a so-called “shutter mode” in which the particles are arranged to move laterally within the display so that the display has one display state which is substantially opaque and one which is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode.
However, the environments in which an encapsulated electrophoretic display can be used is determined, at least in part, by the characteristics of the materials used to form the walls of the microcapsules present in the display, and prior art microcapsules do have some limitations in this regard. The aforementioned applications Ser. Nos. 10/063,803, 10/063,236 and 10/063,655 describe formation of microcapsule walls by coacervation of gelatin and acacia, followed by cross-linking with glutaraldehyde. The resulting microcapsules have an operating temperature range of about +10 to +60° C., may burst at temperatures near the upper end of this range, and are sufficiently sensitive to humidity that the optical performance of the electrophoretic displays is adversely affected at combinations of high temperature and high humidity such as might be encountered in a tropical rain forest environment. Although it might at first appear that such microcapsules could be made to operate at higher temperatures simply by increasing the thickness of the microcapsule wall, increased wall thickness may result in poorly packed films of the microcapsules and/or less deformable microcapsules, and both these effects are disadvantageous in electrophoretic media, as discussed in more detail below. Accordingly, there is a need for improved microcapsule wall materials to expand the operating limits of such electrophoretic displays. In particular, there is a need for improved microcapsule wall materials which will permit electrophoretic displays to operate satisfactorily at extreme temperature and humidity, and thus meet the high performance needs of military and commercial mobile device applications.
However, the search for new microcapsule wall materials useful in electrophoretic and similar displays is complicated by the need for the material to meet the numerous requirements necessary in practical production of such displays. Among the requirements are:
(a) The encapsulation procedure must be reproducible and manufacturable, involve inexpensive raw materials, and yield capsules that are totally impermeable to their contents;
(b) The microcapsules must be amenable to coating. While the properties of a microencapsulated dispersion that allow facile, uniform coating are not entirely understood, one property that is important is flexibility of the capsule wall. If the wall is too rigid, the coating suspension shows severe shear-thickening rheological behavior, and is either impossible to coat because of hopper jamming or yields very non-uniform coatings. Flexibility of the capsule wall also allows closer packing in the coating, and thus yields displays with improved optical properties;
(c) The capsule wall must have mechanical, optical, and electrical properties that allow the construction of a durable display with rapid response at low driving voltages. In particular, the shell must be tolerant to mechanical deformation (this is especially important for flexible display applications) and must not be appreciably colored or opaque. Also, the electrical resistance of the shell wall material must be high; a capsule wall with poor electrical properties can short out the display; and
(d) The capsule wall must maintain its properties over a wide range of operating conditions. The response of the capsule to changes in humidity is especially problematical, since it has been found to be difficult to achieve simultaneously all of the characteristics listed above with a capsule wall composition whose electrical conductivity is sufficiently insensitive to high ambient humidity. Improvements in the environmental sensitivity of the capsule wall represent a major contribution to the robustness of the display.
Furthermore, most microencapsulation techniques known in the literature are intended for controlled release of the capsule contents, so that the microcapsule is intended to break or become selectively permeable in use. Hence, materials developed for other types of microcapsules may not be useful for microcapsules to be used in electrophoretic displays, where capsules must perform in ways that are highly atypical, in that they are intended to provide permanent encapsulation of their contents.
As described in the aforementioned MIT and E Ink patents and published applications, a microencapsulated electrophoretic medium is typically formed by mixing microcapsules with a solution containing a polymer binder, laying down a layer of the resultant microcapsule/binder solution mixture on a substrate, and drying the layer to produce an electrophoretic medium in which the microcapsules are embedded in a layer of the polymer binder. The substrate bearing the electrophoretic medium is then typically laminated, using a lamination adhesive to a backplane arranged to apply drive voltages to the medium. The binder improves the mechanical integrity of the layer of microcapsules, and may improve the adhesion of the microcapsules to the substrate on which they are deposited. It has now been found that the sensitivity of electrophoretic media to humidity can be significantly reduced by modifying the binder and/or the lamination adhesive rather than the material used to form the microcapsules walls.
It is also desirable to reduce the operating voltage of microencapsulated electrophoretic displays. Considerable progress has already been made in this regard; some of the early displays described in the aforementioned E Ink and MIT patents and applications needed to be operated at 90 V, whereas similar displays can now operate at only 15 V. However, further reduction in operating voltage is still desirable, because reducing the operating voltage reduces the energy consumption of the display, an important factor in displays intended for portable devices. Also, when it is desired to drive a display using dry cells or similar small batteries, which only generate (say) 1.5 to 6 V DC, even operating a display at 15 V requires the provision of special circuitry to step up the DC voltage produced by the battery to that required by the display. If the operating voltage of the display could be reduced to that produced by the battery, this circuitry could be eliminated and the cost of the display reduced.
As already mentioned, in an encapsulated electrophoretic display the microcapsules which form the electrophoretic medium are typically enclosed in a binder. The microcapsules/binder layer is typically sandwiched between two electrodes (or, in some cases, between an electrode and a non-electrode support member, a movable electrode being moved over the support member to address the display), it normally also being necessary to include a layer of a lamination adhesive between the electrodes to ensure the mechanical integrity of the display. A potential difference is applied between the electrodes to address the display. Since the switching of an electrophoretic medium is dependent upon the electric field across the medium, the operating voltage required by a display can be reduced by reducing the thickness of the microcapsules/binder layer, since a thinner layer enables the same electric field, and hence the same electro-optic response of the microcapsules, to be produced at a lower operating voltage.
A further problem with some electrophoretic displays is the phenomenon known as “self-erasing”; see, for example, Ota, I., et al., “Developments in Electrophoretic Displays”, Proceedings of the SID, 18, 243 (1977), where self-erasing was reported in an unencapsulated electrophoretic display. When the voltage applied across certain electrophoretic displays is switched off, the electrophoretic medium may reverse its optical state, and in some cases a reverse voltage, which may be larger than the operating voltage, can be observed to occur across the electrodes. It appears (although this invention is in no way limited by this belief) that the self-erasing phenomenon is due to a mismatch in electrical properties between various components of the display; in particular, in the case of an encapsulated electrophoretic display, it appears that the phenomenon is due to a mismatch in electrical properties between the internal phase of the microcapsules and the polymer layer, namely the microcapsule walls, which is in electrical series with this internal phase. Obviously, self-erasing is highly undesirable in that it reverses (or otherwise distorts, in the case of a grayscale display) the desired optical state of the display.
Another problem sometimes encountered with encapsulated electrophoretic displays is that, after the display has been operating for an extended period, the electrophoretic particles may tend to stick to the interior surfaces of the microcapsules, thus ceasing to move when an electric field is applied to the display and the optical contrast between the optical states of the display.
The present invention seeks to provides capsule wall materials, capsules, and encapsulated electrophoretic media and displays, in which the aforementioned problems are reduced or eliminated, and which thus expand the operating range of electrophoretic displays. The capsule wall materials provided by the present invention may be useful for encapsulation of materials other than electrophoretic media, for example pharmaceuticals.